Thanks to significant developments in digital technologies, digital products not only have been continuously improved in performance and function, but also have been more and more extensively used as their selling prices decline. Nowadays, digital products can be found practically everywhere in our daily lives. Digital still cameras, for example, have the advantageous features of real-time display and digital storage, the latter of which supersedes the traditional photographic films and allows users to more conveniently screen the pictures taken than what is possible with a conventional camera, not to mention the subsequent image processing or sharing that can be done with far greater ease. Because of that, digital still cameras have come to be hot-selling items in the digital consumer product market and have, in recent years, been so popular among consumers that there is almost nobody without one. As the market for digital still cameras rapidly expands, huge business opportunities have emerged for camera accessories.
In order to obtain images with various visual effects, the user of a traditional film-based camera would typically resort to filtering lenses of different effects, such as a soft focus lens, a star filter, a polarizing filter (or better known as polarizer), a neutral density filter (ND filter), and various color filters. Referring to FIG. 1, a filter 21 can be connected to one end of a camera lens 20 and assembled to a camera 30 along with the camera lens 20 so that, when the camera 30 is used to take pictures, images with the desired special effect can be obtained. With the advancement of image processing software, however, the aforementioned filters have been gradually replaced by the powerful digital technologies. But this is not true of polarizers and ND filters. An ND filter is a filter for reducing the passage of light. Generally, ND filters are divided into different sets of specifications according to the percentage of light that is allowed to pass, and one or more ND filters of different specifications can be connected to one camera lens to suit practical needs. Once the amount of light passing through the camera lens is properly reduced, pictures can be taken where there is a strong light source, or special photographic techniques (e.g., swinging a black board in front of a camera to achieve uniform exposure in a high contrast environment) can be applied. A polarizer, on the other hand, is a filter based on the principle of polarization of light and has grate-like crystals arranged in a parallel manner. By rotating a polarizer, the angle of its grate-like crystals can be adjusted to regulate the passage of light having a specific polarization angle. Once connected to a camera lens and rotated to an appropriate angle, a polarizer can effectively eliminate surface reflections from the object being photographed (e.g., a water surface, a glass cabinet, or the surface of a leaf of a plant) and thereby produce the desired photographic effect.
As an ND filter of specific specifications corresponds to a specific percentage of light passage, a consumer must buy and carry ND filters of different specifications and choose whichever is suitable for the light intensity of the photographing environment. However, not only is the purchase of plural ND filters costly, but also it is very inconvenient having to carry all the ND filters around. Moreover, the need to frequently mount and dismount the ND filters complicates camera operation. While the conventional polarizers are also capable of reducing the passage of light to some extent, they are, after all, not designed specifically for that purpose and therefore cannot be used in place of ND filters. Besides, it is not uncommon that a polarizer has to be used in conjunction with ND filters of different specifications in order to photograph at different ambient light intensities.
To solve the aforementioned drawbacks of the prior art, an optical filter assembly known as an “adjustable ND filter” was developed and sold in the market. The adjustable ND filter includes two circular frames which are rotatable relative to each other and are each provided therein with a lens. When the circular frames are rotated relative to each other, the lenses are rotated relative to each other, too. In an embodiment of the adjustable ND filter, each of the lenses is a polarizer. By rotating each circular frame, the polarization angle of the corresponding polarizer can be adjusted to change the included angle between the polarization angles of the two polarizers, thereby regulating the percentage of incident light that is allowed to pass. Products employing the aforesaid technical solutions grabbed photography aficionados' attention and received positive feedbacks immediately after they hit the market, but the inventor was not satisfied and, despite the fact that the “adjustable ND filter” is greatly improved in terms of convenience and utility as compared with the traditional filters, sought to make further improvements to benefit photography enthusiasts. By using the adjustable ND filter personally and consistently and exchanging experiences with other photographers, the inventor has found that, although the adjustable ND filter allows its user to freely adjust the passing percentage of incident light and can filter out certain polarized light as can a polarizer, a user sometimes still feels the need to use only one of the functions (e.g., the function of a polarizer to eliminate reflections) or use one of the filters in combination with another lens (e.g., a protective lens). Since the two circular frames of the “adjustable ND filter” are not configured to be disassembled and reassembled at will, a user wishing to use only one of the functions of the adjustable ND filter tends to remove the entire adjustable ND filter from a camera lens and mount a traditional polarizer to the camera lens instead. That is to say, the design of the “adjustable ND filter” lacks variability and flexibility in the use of its lenses and hence still leaves room for betterment.
According to the above, the traditional filters are disadvantageous with regard to purchase costs, portability, and convenience of use. While the inventor has successfully developed an optical filter assembly known as the “adjustable ND filter” which features far more ease of use and significantly higher practicality than the traditional filters, this optical filter assembly is not perfect yet. The issue to be addressed by the present invention, therefore, is to design an optical filter assembly which not only maintains the convenience and usefulness of the “adjustable ND filter”, but also allows the lenses in the optical filter assembly to be flexibly used in various combinations.